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JavaScript transformation

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Sebastian McKenzie @sebmck Web Content Optimisation @ CloudFlare

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JavaScript transformation

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JavaScript transformation myOldWeirdJavaScript(“whatever”); myNewTransformedJavaScript(“yay!”);

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History

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How?

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Source code var foo = function foo() { return bar; };

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{ type: "Program", body: [{ type: "VariableDeclaration" kind: "var", declarations: [{ type: "VariableDeclarator", id: { type: "Identifier", name: "foo" }, init: { type: “FunctionExpression", id: { type: “Identifier”, name: “foo” }, params: [], body: [{ type: "BlockStatement", body: [{ type: "ReturnStatement", argument: { type: "Identifier", name: "bar" } }] }] } }] }] } AST

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AST Variable Declaration Program Variable Declarator Identifier Function Expression Block Statement Return Statement Identifier

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Transformer Manipulates AST Parser Turns code into an AST Generator Turns AST back into code

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Parser Transformer Generator

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Function Declaration Block Statement Return Statement Program Variable Declaration Variable Declarator Identifier Function Expression Block Statement Return Statement Identifier Traversal Visitor

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Replacement [x, y] = calculateCoordinates();

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Replacement var _ref = calculateCoordinates(); x = _ref[0]; y = _ref[1];

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Replacement doSomething([x, y] = calculateCoordinates());

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Replacement doSomething(var _ref = calculateCoordinates()); x = _ref[0]; y = _ref[1];);

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Replacement var _ref; doSomething((_ref = calculateCoordinates(), x = _ref[0], y = _ref[1], _ref));

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Removal left + right; Right Left Binary Expression

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Removal left +; Left Binary Expression

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Removal left; Left

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Uses • Transpilation • Application optimisation • Browser compatibility • Minification • Obfuscation • Hot reloading • Code coverage • Language experimentation • Conditional compilation • Dynamic polyfill inclusion • Module mocking • Code linting • Execution tracing • Intellisense • Profiling • Refactoring • Dependency analysis • Instrumentation • Module bundling • …

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• Transpilation (ie. ES2015 to ES5) • Application optimisation • Browser compatibility • ??? ✨

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• Additional standard lib methods • Arrow functions • Block scoping • Classes • Collections • Computed properties • Constants • Destructuring • Default and rest parameters • Generators • Iterators and for…of • Modules • Promises • Property method and value shorthand • Proxies • Spread • Sticky and unicode regexes • Symbols • Subclassable built-ins • Template literals • Better unicode support • Binary and octal literals • Reflect API • Tail calls

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ES2015 Arrow Functions var multiply = (num) => num * num;

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ES2015 Arrow Functions • Implicit return for expression bodies • “Inherits” arguments and this binding • Cannot new it • No prototype

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Implicit return for expression bodies var multiple = (num) => num * num; // turns into var multiply = function (num) { return num * num; };

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ES2015 Arrow Functions • Implicit return for expression bodies • “Inherits” arguments and this binding • Cannot new it • No prototype ✓

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arguments and this var bob = { name: “Bob” friends: [“Amy”], printFriends() { this.friends.forEach(f => console.log(this.name + " knows " + f) ); } };

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arguments and this var bob = { name: “Bob”, friends: [“Amy”], printFriends() { var _this = this; this.friends.forEach(function (f) { return console.log(_this.name + " knows " + f); }); } };

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ES2015 Arrow Functions • Implicit return for expression bodies • “Inherits” arguments and this binding • Cannot new it • No prototype ✓ ✓

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no new var foo = () => {}; new foo; // should be illegal!

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no new function _construct(obj) { if (obj.name === “_arrow”) throw new Error(“nope”); return new obj; } var foo = function _arrow() {}; _construct(foo);

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no new function _construct(obj) { if (obj._arrow === “_arrow”) throw new Error(“nope”); return new obj; } var foo = function () {}; foo._arrow = true; _construct(foo);

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• Implicit return for expression bodies • “Inherits” arguments and this binding • Cannot new it • No prototype ✗ ES2015 Arrow Functions ✓ ✓

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no prototype var foo = () => {}; foo.prototype; // should be undefined!

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no prototype function _getPrototype(obj) { if (obj._arrow) { return undefined; } else { return obj.prototype; } } var foo = function () {}; foo._arrow = true; _getPrototype(foo);

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no prototype var bar = “prototype”; var foo = () => {}; foo[bar];

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no prototype function get(obj, key) { if (key === “prototype”) { return obj._arrow ? undefined : obj.prototype; } else { return obj[key]; } } var bar = “prototype”; var foo = () => {}; get(foo, bar);

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Do not use transpilers as a basis to learn new language features

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Compile-time vs Runtime function square(num) { return num * num; } square(2); square(age);

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JSX var foo =
{text}
;

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JSX Constant Elements function render() { return
; }

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JSX Constant Elements var foo =
; function render() { return foo; }

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JSX Constant Elements var Foo = require(“Foo”); function createComponent(text) { return function render() { return {text}; }; }

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JSX Constant Elements var Foo = require(“Foo”); function createComponent(text) { var foo = {text}; return function render() { return foo; }; }

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Precompiling tagged templates import hbs from “htmlbars-inline-precompile"; var a = hbs``;

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import hbs from “htmlbars-inline-precompile"; var a = Ember.HTMLBars.template(function() { /* crazy HTMLBars template function stuff */ }); Precompiling tagged templates

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• Shouldn’t rely on preprocessing for functionality • YOU can make assumptions about your code • JS engine can’t be more lenient

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Named function expressions var f = function g() {}; typeof g === “function”; // true f === g; // false https://kangax.github.io/nfe/#jscript-bugs

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What’s the solution?

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export function FunctionExpression(node, print) { if (!node.id) return; return t.callExpression( t.functionExpression(null, [], t.blockStatement([ t.toStatement(node), t.returnStatement(node.id) ])), [] ); } Automate it!

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Result var f = function g() {}; // becomes var f = (function () { function g() {} return g; })();

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Emojification Emojification

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ES2015 • Unicode code point escapes • var \u{1F605} = “whatever"; • Emojis

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How? $ npm install babel babel-plugin-emojification $ babel --plugins emojification script.js

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myOldWeirdJavaScript(“whatever”); myNewTransformedJavaScript(“yay!”);

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